1. Technical Field
Embodiments of the present invention relate to a receiver device in an optical communication system, more particularly to a receiver device employed in an optical communication system that utilizes a coherent method.
2. Description of the Related Art
The current optical communication system uses an IM-DD method with which the transmitter unit converts an electrical signal into an optical signal and then directly applies intensity modulation (IM) before transmitting the signal, and the receiver unit applies direct detection (DD) on the received optical signal.
While the capacity of data transmissions for broadband total communication networks in future applications is expected to range from hundreds of gigabytes per second to tens of terabytes per second or even higher, the IM-DD method may not be easy to apply systems having a transmission capacity of 10 Gb/s or higher, because it does not allow phase modulation, so that the efficiency of bandwidth use is low, and the reception sensitivity is also low.
Due to such requirements, coherent optical communication is again gaining interest, as it allows improved reception sensitivity by mixing a local oscillator signal with a received signal and allows improved bandwidth efficiency through phase modulation.
In order to utilize coherent optical communication in practical applications for high-capacity, long-distance communication, the spectral linewidth of the semiconductor laser used for the optical sources in the transmitter and local oscillator must be very narrow, and the central frequency must be stable.
Also, in order to perform coherent optical communication in a stable manner, the frequency of the received optical signal and the frequency of the local oscillator signal must be the same, with precise phase control required between the two signals.
However, due the surrounding environment, physical properties, etc., it is virtually impossible to prepare two optical sources (e.g. laser diodes) that emit completely identical frequencies. Therefore, a frequency offset occurs between the local oscillator signal and the received optical signal.
Also, the phase may randomly change due to the properties of the laser diode having a linewidth, resulting in the occurrence of phase noise which distorts signals.
Therefore, the receiver device in a coherent optical communication system may require an additional technique for precisely controlling the frequencies and phases of two different optical sources or for compensating the distortions.
To address the problems above, the self-homodyne coherent optical communication system was proposed. The self-homodyne coherent optical communication system has the transmitter device divide the optical carrier and provide the optical source that will be used for the local oscillator in the coherent receiver.
However, the existing self-homodyne coherent optical communication system requires additional optical fibers for transmitting the optical source from the transmitter device for use as the local oscillator signal, so that the efficiency is lowered and the cost is increased due to the installing of the optical fibers.